Corrosion resistant coated steel members and method of making same



United States ice Jr., Drexel Hill, Pa., assignors, by mesne assignments,

to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania N Drawing. Filed June 20, 1956, Ser. No. 592,552

2 Claims. (Cl. 117-50) This invention relates to coated steel members, more particularly to such members which are protected from corrosion.

Among the objects of the present invention is the provision of novel coated steel members which are simple and inexpensive to make and yet are extremely resistant to corrosion.

Additional objects of the present invention include the provision of a novel method for making such members.

The above as well as still further objects of the present invention will be more clearly understood from the following description of several of its exemplifications.

Plain carbon steels such as ordinary SAE 1010 steel, or the conventional Blackplate sheets that are used in the canning industry, are relatively inexpensive and simple to fabricate and use in practically any desired configuration. Unfortunately, these steels =are extremely vulnerable to corrosion. In fact, unless the steel is protected as by a film of oil, it will usually corrode even before it can be delivered by the manufacturer to the consumer.

However, by reason of the low cost and extreme versatility of these steels, many different types of treatment have been developed to reduce their corrosion so that their use can be extended to such fields as canning and the like. Perhaps the most widely spread corrosion-resistant treatment is the application of a tin plating. Although the plating of tin has proven to be very satisfactory in most respects, it is fairly expensive and requires in this country that the tin be imported from abroad.

According to the present invention, very effective corrosion resistance can be imparted to members made of the above steels, particularly where these members are to come into body-engaging contact, by applying a coating of an in situ formed combination of hydrated chromium oxides containing about 20 to 60% chromium by weight, about 40 to 95% of the chromium by weight being trivalent, the remainder being hexavalent, and the coating weighing about to 200 milligrams per square foot of surface that it covers. The steel is preferably subjected to a grain-boundary etch, or is oxidized so that its surface is covered with a film of uniformly adherent iron oxide, before the coating is applied.

A can of lubricating oil or roasted coffee beans can have its inner surface covered with the above coating to prevent rusting prior to filling as well as when filled with oil or coffee beans; the outer surfaces of the can can also have the same type of combined chromium oxide coating which provides an excellent substrate for subsequent lithographic coatings, varnishes, lacquers, enamels and other organic coatings.

Instead of applying the coating of the present invention to the can after it is manufactured or after it is filled and sealed, it is simpler to make the can from steel sheets that have the coating applied while they are being produced. This permits the coated sheets to be stored, if

desired, and even shipped over substantial distances without requiring any supplemental treatment to prevent them from becoming unmarketable or unsightly as a result of corrosion.

The sheets are readily coated by first making sure they are very clean and then passing them through an aqueous solution of chromic acid and a reducing agent which is compatible with the chromic acid. This compatibility means that the chromic acid and the reducing agent, even though both are present in the solution, will not in the appropriate dilution react rapidly with each other and will not form a visible precipitate while the solution is being contacted with the steel.

The steel sheet wet with the solution is then dried at a temperature above 212 degrees F. During this drying, the chromic acid reacts with the reducing agent and becomes partially reduced. As a result, the final coating is a combination of hydrated chromium oxides containing about 20 to 60% chromium by weight.

An inordinately high degree of corrosion resistance is obtained when 40 to 95% by weight of the chromium in the final coating is in What corresponds tothe trivalent condition, the remainder being hexavalent, and the coat ing itself weighs about 10 to 200 milligrams for every square foot of surface that it covers. Coatings of less weight give less protection, although coating weights need not be more than about 30 milligrams per square foot to give all the protection that is needed.

If the reducing agent is omitted from the coating bath, dried coatings of the above weight range will not show the desired trivalent chromium content. They also tend to be somewhat deliquescent and therefore inferior.

Typical reducing agents suitable for use in connection with the present invention are organic polyalcohols such as sugars, including invert sugar, sucrose, dextrose, glycol and polyethylene glycols, glycerine, mannitol, sorbitol, triethanolamine, hydroxylamine salts such as its sulfate and hydrochloride, phosphorous acid, and potassium iodide.

Care should be taken to limit the amount of watersoluble material that is included in the coating. Although as much as 5 or 6% of such materials can generally be tolerated, it is preferred to use reducing agents that do not leave such materials in the final product. Potassium iodide is not a preferred type of reducing agent inasmuch as it is carried through as Water-soluble potassium compounds after reducing the chromic acid. Phosphorous acid is a better reducing agent even though it is oxidized to phosphoric acid because the drying operation converts such phosphoric acid to water-insoluble phosphates. The most effective reducing agents appear to be the organic polyalcohols since they are readily oxidized at the higher temperatures and are fairly stable at the lower temperatures.

As indicated above, the coatings of the present invention are preferably applied to a steel that has had its surface subjected to a grain-boundary etch or covered with an oxide film. The etching may vary from an insignificant amount to a heavy etch that removes to 1000 milligrams of metal per square foot of steel surface. With etching of more than about 400 milligrams per square foot, the best results are not obtained unless the mixed chromium oxide coating weighs more than about 30 milligrams per square foot. The oxidation can be effected by merely attacking the surface with a reagent that converts it to oxide with substantially no removal of metal.

For etching the steel surface can be contacted with aqueous nitric acid having a concentration of from 1 to 20% HNO by weight for a period of from 2 to 70 seconds at a temperature of from 60 to F. Aqueous solutions of ammonium persulfate, picric acid or ferric nitrate are also effective and react similarly to nitric acid solutions. For example, an aqueous solution containing 4% of ammonium persulfate will, when applied at 75 F for 15 seconds, remove approximately 380 milligrams of metal per square foot and provide a very satisfactory surface for the chromium oxide coating. A 1 /z% picric acid solution in water at 155 F. for 12 seconds removes about 370 milligrams of metal per square foot and is also very satisfactory. Aqueous ferric nitrate in a concentration of about 5% based on anhydrous material, used at 80 F. for 15 seconds removes about 400 milligrams per square foot and also leaves an excellent pretreatment surface. The oxidation can be accomplished with hot concentrated aqueous solutions of sodium hydroxide containing sodium nitrate as used in standard bluing operations. A concentration of 39% NaOH and 2% NaNO used at 280 F. is very effective although any bluing treatment appears to be suitable.

It is not desirable to have oxide pretreatment films that are loose or non-adherent; the oxide produced should be continuous and adherent to the metal surface. However, such films as ordinary corrosion do not interfere with the application of the coating since loose portions are readily removed as by a conventional pre-cleaning operation.

The proportion of reducing agent to chromic acid should be insufiicient for the complete reduction of all the chromic acid to trivalent condition. The minimum amount of reducing agent is somewhat below that which will stoichiometrically reduce the lowest proportion of the chromium, inasmuch as some of the chromium is reduced during the drying operation even if the reducing agent is not present in the bath. Chromic acid solutions of any concentration can be used and the coating weight adjusted by controlling the amount of solution that is left on the metal surface when it is being dried.

The final heat treatment is somewhat more effective if carried out at temperatures substantially above 212 degrees F. Temperatures between 250 degrees F. and 350 degrees F. are preferable and provide the most corrosionresistant and adherent forms of coating. At about 450 degrees F. and higher, however, the coating appears to be adversely affected.

In accordance with the above, cans can be made from unfabricated full bright finished sheet steel mils thick by subjecting the sheet to the following operations:

A. Clean the sheet cathodically in an aqueous solution containing 16 grams KOH per liter using a current density of amperes per square foot of cathode at 140160 F. for 10 seconds.

. Cold water rinse.

. Clean anodically in an aqueous solution containing 16 grams KOH per liter using a current density of 15 am-t peres per square foot of anode at l40-160 F. for 10 seconds.

. Cold water rinse.

. Flood with a passivity-preventing /2 aqueous H 80 solution by weight 1-2 seconds at 80 F.

. Cold water rinse.

. Spray with an aqueous solution containing 2% nitric acid, 80 F., 8 second using a spray pressure of about 6 pounds per square inch.

. Cold water rinse.

Desmut by brushing in water to remove loose or non-adherent reaction products including any developed in G.

1. Flood with an aqueous solution containing 4% chromic acid and 1.3% cane sugar at 75 F. for 2 seconds.

K. Roll through rubber rolls wetted with the flooded solution.

L. Cure by passing the resulting filmed metal through a drying unit having a set of ceramic gas burners heated red hot by burning gas, a five second exposure to the incandescent units being used, and the metal reaching a temperature of 300 to 350 F.

The addition of a wetting agent such as 0.005% of the product made by condensing 3 mols of ethylene oxide with p-(n-octyl) phenol, as described in U.S. Patent No. 2,115,192, improves the Wetting in step I.

If the metal is in the form of an elongated strip, it may be coiled up directly after step L.

Hm can we The same coated sheet can be used for making both the top and bottom covers of a can, although in some cases different thicknesses of metal can be used in the different portions, so that separate coated sheets are required.

After the can body is secured together, the cover on one end can be applied in any convenient manner such as the one generally used in the industry. The can can then be filled and the remaining cover applied with the usual precautions in the case where the contents have to be sterilized or heated, or kept in a special atmosphere such as under evacuation or superatmospheric pressures. The cans having the coatings of the present invention are particularly useful in storing such materials as dried foods, nuts, spices, dough mixtures, etc., as well as the above-mentioned coffee and lubricating oil. Wet-packed foods or other materials can also be stored in the cans of the present invention, but here it is desirable to have an organic covering layer (enamel, lacquer, varnish) applied over the coating of the present invention on the inside surface of the can.

The coatings of the present invention contribute a substantial amount of increased adhesion and wear resistance for such organic coatings as well as resistance to cor: rosion. Suitable organic layers are those usually loosely referred to in the art as enamels, sanitary enamels or lacquers such as the oleo-resinous phenolic or vinyl resin varnishes. Particularly effective forms of such organic type coating are described in U.S. Patents Nos. 2,231,- 407, 2,299,433, 2,479,409 and 2,675,334. Such top coatings will even further reduce corrosion as well as increase the wear resistance and lower the contamination of the can contents by the oxides.

The advantages of the present invention are contributed to any plain carbon steel, that is steel that contains no more than about 2% of alloying metals. They can have a carbon content varying from extremely low values, 0.05% or even less, to as much as 1.4% or higher. The phosphorus and sulphur contents can range from substantially zero up to several tenths of a percent. Generally phosphorus maxima are about 0.15% and sulphur maxima about 0.3%. These materials include the steels ordinarily considered as carbon steels (SAE 1010 to 1095), free cutting steels, plain carbon tool steels, in-- cluding those that have up to several percent of silicon, and casting metals.

The inorganic mixed chromium oxide coating of the present invention is even further improved if after the final high temperature drying it is subjected to a quench that rapidly reduces its temperature at least about 25 F. Any liquid appears to be suitable for this purpose, and plain or tap water is very effective. The addition of 0.03 to 1.0% CrO in the quenching Water even further improves its effectiveness.

The mixed chromium oxide is also improved by including with these oxides a small amount of an oxide of a metal such as molybdenum that is in the same periodic group as chromium. Such supplemental oxide can be added to the chromic acid bath as water-soluble oxides, acids, or as water-soluble salts. It is preferred to add molybdenum as M00 The advantages of the supplemental oxides are felt when they make up as much as /3 the weigh-t of the final coating, although it is preferred to use from 5 to 20%. The combined oxide coatings show particularly good adhesion to thestecl with or without a previous oxidation, as Well as better adhesion to an organic top layer. Similarly reducible compounds of other metals such as titanium and vanadium can also be used. In general, it is desirable to diminish the proportion of reduced chromium oxide in the final coating, when a supplemental oxide is used, to not over approximately- 75%.

The inclusion of 1 to 5% of insoluble or slightly soluble ohromate of such divalent metals as zinc, calcium and strontium also improves the quality of the coating in a similar manner. This effect is obtained with or without the supplemental oxide.

The improvements contributed by the quenching as described above are also produced with the coatings that have the supplemental oxides, or the slightly soluble chromates, or both.

The above coating techniques can be readily carried out either in a batch process or continuously. They may, for example, be added at the end of a standard sheet steel production line. In fact, the coating treatment of the present invention can be carried out with the steel moving at a relatively high speed through the necessary treating stations. Under some conditions, particularly Where the steel is moving very rapidly through a chromic acid bath, it is desirable to have a wetting agent present in the bath. This enables the bath liquid to more rapidly and uniformly wet the surface of the metal. Wetting agents of any type can be used so long as their wetting action is not completely destroyed by the oxidizing action of the bath. Anionic, cationic or nonionic types of wetting agent used in amounts of about 0.001 to 0.2% by weight of the bath are effective to keep from developing coating irregularities apparently due to air bubbles trapped on the metal while moving through the chromic acid bath. A highly elfective example of wetting agent is the polyoxyethylene ether of alkylated phenols such as those produced by condensing dodecyl phenol with twelve molecules of ethylene oxide. Reference is also made to U.S. Patents 1,970,578 and 2,085,706 for more specifically disclosed wetting agents that are suitable.

Another example of the coating process of the present invention is as follows:

A. Sheets of 14 mil thick SAE 1010 steel are cleaned by immersing for five seconds in a 180 F. aqueous solution of 5% disodium phosphate and 3% sodium carbonate.

. The cleaned sheets are rinsed with water at 70 F.

. Immerse the rinsed sheets in solution of sulfuric acid in water for one second at 70 F.

. Rinse again in water at 70 F.

. Subject the resulting sheets to a uniform action of jets of an aqueous solution containing 1.2%nitric acid at 80 F. for two seconds, the jets impinging at a velocity of ten feet per second.

. Rinse the acid treated sheets in cold water at 70 F.

. Pass the sheets between rotating brushes having stainless steel bristles, to remove loose material.

H. The rinsed sheets are sprayed with an aqueous solution of 3 /2% chromic acid, 1% pentaethylene glycol and 0.1% of the wetting agent made by condensing 3 mols of ethylene oxide with p-(n-octyl) phenol, as described in U.S. Patent No. 2,115,192. This solution was prepared in the mixing nozzle by supplying it with two separate streams, one being an aqueous solution of the chromic acid, and the other an aqueous solution of the remaining ingredients.

I. The sheets carrying the above solution are passed through an air oven, the inside of which is held at 800 F., the surface of the sheets reaching a temperature of 375 F.

J. The hot sheets are quenched water at 70 F.,

and then permitted to dry.

The dried product is extremely resistant to corrosion, particularly if coated with an acrylate lacquer or even a thin film of methylmethacrylate resin. Other resins such as those made with ester-type Waxes, carnauba wax for example, are also very effective.

The oxide-coated steel of the present invention is further protected against corrosion by applying to the coating a film of an oil such as a paraflin or a glyceride oil. Thus mineral oil or palm oil can be applied in very minute quantities (0.5 milligram or more per square foot) as by conventional electrostatic coating techniques and enhances the corrosion resistance of the oxide coatings as well as reduces friction to simplify fabrication operations such as stamping, bending, etc.

Under some conditions the nitric acid treatment of the steel does not produce the desired effect unless the steel is subjected to a preliminary activation. Under these conditions the steel appears to be passive, and any chemical attack as by very dilute sulphuric acid or even mechanical working of the steel will suflice to activate it. The addition of /2 to 2% urea to the nitric acid bath tends to suppress NO formation, thereby reducing the tendency for iron to be oxidized to ferric form. The urea is consumed in the process and can therefore be replenished either on a continuous basis or by infrequent additions.

As stated previously, etching and oxidizing with HNO can be effected with concentrations of HNO from 1 to 20% and temperatures from 60 to F. Not only is it desirable from a handling and economic standpoint to work with low concentrations of HNO (less than 5% by weight) and low temperatures (less than 110 F.) but extremely advantageous from an operational standpoint. The lower concentration and temperature provide somewhat more uniform results and greatly minimize the tendency for oxidation of ferrous iron to ferric iron. It should be noted that this oxidation consumes excessive amounts of nitric acid and is accompanied by the generation of large quantities of noxious nitrogen oxide fumes.

Other articles with which the invention is very effective are shown in U.S. Patent 2,773,623, granted December 11, 1956.

The coatings of the present invention can be applied to articles after they are formed as by shaping, stamping, or even casting. When treating such formed articles with the liquid coating solutions, however, care should be taken to see that the layer of solution on the article does not concentrate as by running or dripping to any portion of the article that should be protected. A localized thick layer of solution tends to form on the lower edges of the article under the influence of gravity, but a blast of air can be used to redistribute the thick layer. Alternatively, the articles can be rotated or kept moving so as to keep a thickened film from forming at any location.

Although it is indicated above that coating weights of at least ten milligrams per square foot are required to give the exceptional corrosion resistance for some purposes, such as to anchor organic covering layers, coatings of somewhat lower weight will provide unusually good results.

The Weight of the coating is readily determined by dissolving it in the flux or in a strong alkaline solution such as 20% NaOH in water at F. and weighing the metal before and after. The solution thus made can have its hex-avalent chromium content determined by titration with sodium thiosulfate. The total chromium content can be determined by oxidizing another portion of the solution with sodium peroxide and then again titrating with sodium thiosulfate. Alternatively, the coating can be scraped off, dissolved and analyzed.

The present application is a continuation-in-part of application Serial No. 433,698, filed June 1, 1954, now Patent No. 2,773,623, granted December 11, 1956. That application is in turn a continuation-impart of Serial No. 277,286, filed March 18, 1952, now Patent No. 2,768,- 103, granted October 23, 1956; Serial No. 278,481, filed March 25, 1952, now Patent No. 2,768,104, granted October 23, 1956; and Serial No. 371,427, filed July 30, 1953, now Patent No. 2,777,785, granted January 15, 1957.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A plain carbon steel the surface of which is etched With ammonium persulfate, the etched surface having a corrosion-inhibiting coating of an in situ formed combination of hydrated chromium oxides containing about 20 to 60% chromium by weight, about 40 to 95% of the chromium by weight being trivalent, the remainder being hexavalent, and the coating weighing about 10 to 200 milligrams per square foot of surface that it covers.

2. A plain carbon steel the surface of which is picric acid etched; the etched surface being covered with a corrosion-inhibiting coating of an in situ formed combination of hydrated chromium oxides containing about 20 to 60% chromium by Weight, about 40 to 95% of the chromium by Weight being trivalent, the remainder being hexavalent, the coating weighing about 10 to 200 milligrams per square foot of surface that it covers.

References Cited in the file of this patent UNITED STATES PATENTS Age 

2. A PLAIN CARBON STEEL THE SURFACE OF WHICH IS PICRIC ACID ETCHED; THE ETCHED SURFACE BEING COVERED WITH A CORROSION-INHIBITING COATING OF AN IN SITU FORMED COMBINATION OF HYDRATED CHROMIUM OXIDES CONTAINING ABOUT 20 TO 60% CHROMIUM BY WEIGHT, ABOUT 40 TO 95% OF THE CHROMIUM BY WEIGHT BEING TRIVALENT, THE REMAINDER BEING HEXAVALENT, THE COATING WEIGHING ABOUT 10 TO 200 MILLIGRAMS PER SQUARE FOOT OF SURFACE THAT IT COVERS. 